(1) Field of the Invention
The present invention relates to a photopolymerizable resin composition having an improved adhesion to a metal surface, especially one suitable for use as a photoresist for fabrication of a printed circuit board.
(2) Description of the Related Art
Ordinarily, fabrication of a printed circuit board by using a photoresist is carried out by the steps of (1) laminating a photoresist on a substrate such as a copper clad epoxy-glass board, (2) irradiating the substrate with active rays through a pattern mask film to form an image, (3) selectively dissolving and removing the uncured portion of the photoresist by using an appropriate developing solvent to expose the copper surface and form a resist pattern, (4) electrolytically plating copper on the exposed copper surface and then electrolytically plating solder, (5) removing the cured resist by using an appropriate solvent to expose the copper surface, and (6) etching the exposed copper surface with an appropriate solvent by using the solder-plated layer as the resist to obtain a printed circuit board in which the solder-coated portion forms a wiring portion.
The important requisite for the photoresist in the above-mentioned fabrication process is that the photoresist is not corroded by a plating solution or the like and the substrate covered with the photoresist is sufficiently protected. Especially, peeling of the cured resist caused at the high-throw solder plating results in intrusion of the plating solution between the cured resist and the copper surface. Consequently the solder is plated on the portion other than the circuit portion and such troubles as short-circuits of conductor lines, broadening of the thickness of conductor lines, and disfigurement of the peripheral edges of conductor lines are caused.
These troubles are due to an insufficient adhesion of the cured resist to the copper surface. Therefore, researches have been made for developing adhesion promoting agents. For example, U.S. Pat. No. 3,622,334 proposes addition of a heterocyclic nitrogen-containing compound such as benzotriazole or benzimidazole to a photopolymerizable resin composition.
However, in the production of a printed circuit board comprising wiring lines having a narrow thickness sufficient to meet recent requirements of high density and high precision, a high-precision pattern cannot be obtained unless the amount of the abovementioned heterocyclic nitrogen-containing compound added is increased. This results in occurrence of other troubles. More specifically, an increase of the amount of benzotriazole or benzimidazole added causes discoloration or fading of a dye used for confirmation of the pattern configuration or reduction of the sensitivity of the photoresist. If the amount of the photopolymerization initiator added is increased for increasing the sensitivity, the section of the resolved pattern comes to have an inverse trapezoidal shape and the resolving power is reduced. Moreover, if the amount of an additive having a low molecular weight is increased, dissolution of the cured resist layer is caused at various steps and the resistance of the resist film is reduced.